| Summary: | The conditions for the hydroxamic acid reaction for determining esterified fatty acids have been modified to control the variable factors involved and to obtain molar absorptivities per ester group for triglycerides, cholesterol esters of long-chain fatty acids, and phosphatides that are equivalent for amounts up to 8 μEq of ester. Control of the amount of water present during the formation of hydroxamates was the most important single factor in obtaining equivalent color values. The accuracy and precision of the method have been well defined by showing that the optical density values for five different ester standards were on the identical straight-line curve. The colored end-products gave identical spectral curves between the wavelengths of 410 mμ and 700 mμ whether they were derived from standard carboxylic acid esters, Folch extracts of rat serum, rat liver, or human serum. A long-chain cholesterol ester must be used as one of the standard esters because of its special solubility characteristics. Cholesteryl acetate cannot be used as a reliable representative in place of a long-chain cholesterol ester.
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